The U.S. Department of Energy (“DOE”) has given priority to the development of transformational technologies that reduce the barriers to natural gas use in vehicles. While natural gas enjoys a significant price advantage over gasoline, there are significant technological and economic barriers to its use. These challenges arise largely from the low volumetric energy density of natural gas compared to liquid phase gasoline. Therefore, the suitable on-board storage of methane becomes an important challenge in developing natural gas-powered vehicles.
Technological approaches for storing natural gas on-board have included the storage of liquified natural gas (LNG), compressed natural gas (“CNG”) and absorbed natural gas (“ANG”). LNG has been commercially used for heavy-duty vehicles, whereas the storage of CNG has been commercialized for light-duty vehicles. The storage of ANG is developmental, with a Department of Energy target storage pressure of 35 bar.
High surface area adsorbents such as zeolites, metal organic frameworks (“MOF's”), covalent organic frameworks (“COF's”), activated carbon and carbon nanotubes have been investigated for storing natural gas. So far, storage capacities of these adsorbents for methane (CH4) at ambient temperature and 35 bar have been much lower than the DOE ARPA-E (Advanced Research Projects Agency-Energy) targets of 12.5 MJ/L volumetric, and 0.5 grams CH4/gram sorbent gravimetric. Thus far, the reported adsorbent capacities at 35 bar have not exceeded 0.29 grams CH4/gram sorbent. However, the adsorbents are far from saturation and have the potential to adsorb higher levels of methane using higher loading pressures that exceed the 35 bar storage pressure.
There is a need or desire for a method for loading and releasing natural gas and other gases (including, for example, carbon dioxide and hydrogen) into adsorbents that maintains higher loading levels of CH4 at a storage pressure of 35 bar.